Thermostructural composite materials are characterized by their mechanical properties which make them suitable for constituting structural elements, and by their ability to retain these properties up to high temperatures. These materials are in particular carbon/carbon (C/C) composite materials comprising reinforcement made of carbon fibers embedded in a carbon matrix, and ceramic matrix composite (CMC) materials comprising reinforcement of refractory fibers, such as carbon or ceramic fibers, embedded in a ceramic matrix.
Because of their properties, thermostructural composite materials can replace metal materials for making parts that are subjected to mechanical forces and that are liable to be exposed to high temperatures.
Thus, proposals have already been made to make pistons for internal combustion engines out of C/C composite material. In addition to withstanding temperature, C/C composite materials have the advantage of density that is much smaller than that of metals, thereby making it possible to reduce moving masses and to increase the maximum speed of rotation of the engine. In addition, the low thermal expansion of the material and its good tribological properties greatly reduce the risk of the piston seizing or binding, thereby making it possible to increase the reliability of the engine and reduce its requirements for lubrication.
Reference can be made to document U.S. Pat. No. 4,909,133 which describes the manufacture of a piston by making an article of fiber reinforcement, or "preform", as a single piece in the form of a knitted "sock" of carbon fibers, impregnating the preform with a resin for molding and densification by means of a carbon matrix by using a liquid process (impregnation with the resin followed by carbonization) and finished off by a gas process (chemical vapor infiltration).
Document DE-A-29 12 786 also describes making a C/C composite material piston by winding a carbon thread and then impregnating with a carbon-containing resin. The outside surface of the piston on its side that is exposed to combustion gases is coated in a protective layer of silicon carbide.
In a C/C composite material piston, it is necessary to ensure that the reinforcement fibers are oriented in directions that are suitable for the forces to which the piston is subjected in operation. The techniques described in the two above-mentioned documents can satisfy that condition, but in a manner that is expensive. They require preforms to be made individually in relatively complex manner: knitting a "sock" or winding a filament over a predetermined path.